studiot

Note again you have avoided a direct question. (That's the line with a question mark that you quoted). As I understand Carnot's crowning glory it was the realisation that "If you have a source of heat you can obtain work from it " or words to that effect. So please provide a proper reference with all important dates, as I requested.

I stand corrected. Thanks. +1

And who, exactly, realised that and when did this Eureka moment arrive ? You have been noted as castigating Carnot many times for his 'belief in caloric'

So rather than leave this thread on a sour note, I think I will post more of my thoughts on the actual subject. Firstly the stubborn factor of 1/2. I don't know why the code writes incorporated it but I do know thet acknowledge that the real wind velocity variest from near zero close to the ground to a maximum near the top of a wall. So taking the max velocity and multiplying it by some factor less than one seems reasonable as an average. The code also describes finding and using the centre of pressure for the actual application of teh wind load to the wall. Now a couple of other members have pointed out Newton's third law say that If the wind applies a force F to the wall then the wall applies an equal but opposite force F to the wind. So where does that leave my analysis ? Over cautious, but safe in engineering terms. Modern codes also stress that they are starting point average simple solutions with no complicating factors. And that the individual situations should take account of special circumstances, using wind tunnels if necessary. What lessons does that leave us with ? Well the wind stream is taken as steady and continuous. This poses difficulties for the idea that a region of higher density builds up near the wall as this cannot continue indefinitely. Eventually as much air must be leaving the vicinity as arrives. So what we can say is the the presence of the wall alters the stream pattern which is best represented as a pressure averaged over the whole face of the wall. This pressure is then converted to a representative force, applied at a suitable point. This point of application is important because Rocks wondered how the wind could blow over a wall. For a wall this collected force applies a moment about the wall foundation. As the wind changes this can cause rocking or oscillation. Such motion can cause the wall to shift over time on its foundation sometimes sufficiently to become unstable.

I'm suprised Tom didn't go back to the realy father of thermodynamics, that ancient Greek who thought the Earth was flat and that there were four elements earth, water, air and heat (fire). So can I quote them to promote my flat earth leanings ? Anyway going back to retrieve quotes from Carnot an his contemporaries it is not suprising the they talekd of 'destroying' heat. After all they thought it to be a substance. And their word for it was caloric, which unfortunately survives to this day in French as Chauler, Portugese and Spanish as calor, etc. Atom was another ancient Greek concept that we now understand quite differently from the original. So what, our knowlesdge and understanding of physics has improved over the millenia so keeping older ideas is only useful if for engineers if it provides an easy route to calculation or to historians.

Nah, it's the air-fuel mixture. Nothing would happen without that. And a lot of work is done on that mixture to raise its temperature, in part of the cycle.

well said. +1 @NTuft Cowan has a very easy transition from Newton to Eulerian to Lagrangian mechanics. Hamill is much more red loodedas the whole book is as its title suggests. But still relatively easy going.

Exactly. and what do you think the working fluid is in a diesel engine ?

Yes I see it at the end of the address bar. But how would my browser know which post I was looking at ? For instance it shows blah-blah-blah/6/#comment-1229858 for every post on page 6.

Very clear thank you. Clear that you have classed all combustion engines as not being heat engines.

All I see is a yellow (ish) box with a date and time popping up. But if you really have found a numbering system that would be marvelous.

Not necessarily. Some chemical reactions evolve or accept either work or heat or both sometimes with a temperature change, sometimes not. You made no answer to my previous post so this is the last time I attempt to help you.

Yes, a groups is simpler and more basic than a field. If you can demonstrate the set elements forming a group, your propsed field can inherit group properties to satisfy field axioms. Field, group, algebra are some of the many words that combine with others into specialist phrases. Here are some useful definitions for you in algebra and number theory.

Not always, no. What about the heat of combustion ? What about latent heat ?

I didn't say that was the reason. It wasn't. And I didn't say they were wrong full stop at any point, not a courtesy to extended to me. I did say they were not completely correct. I also indicated that others in this thread made some good points as well as some less good ones. Yet you singled me out in most of your posts for uncouth personal comments. So yes I will remain a 'baby' if that is the way adults behave these days.

Reference please. My opening article said and I quoted

Most definitely a great summary. +1 We have all missed points here which is why the wind loading design codes offer exactly this. For example CP3, Chapter 5 in the UK offers 1) From the map of standardised wind speeds and modifying tables choose the design wind speed, V, appropriate to the location. V is in m/s 2) Calculate the 'dynamic pressure' , q from the equation q = 0.613V2 N/m2 The coefficient 0.613 is not dimensionless but enjoys suitable units of conversion. This will yield a pressure similar to Seth's calculation, but for different reasons from either his or mine. The history of wind loading in UK structural engineering really starts in 1879 with the Tay Bridge disaster. It is interesting to note that in those days responsibility for meteorolgy rested with The Astronomer Royal, then Sir George Airy. The bridge designer, Sir Thomas Bouch wrote to Airy, requesting a pressure loading figure and received a written reply of 10 pounds per square foot But he hastily revised this fugure to 120 pounds per square foot, following the disaster. Bouch, however collected all the blame. Subsequent to this a whole series of experiments were carried out at the National Physical Laboratory relating loading to building shape and size during the later 1880s and 1890s. This led to the CP3 figure, originally in imperial units along with a bunch of modifying factors. So I disagree that there is nothing worth discussing here, so long is it is not in such inflammatory language. The mechanics of this must be Newtonian so we have all missed points here, myself included and we can all benefit from acknowledging the contributions from all concerned (including those not mentioned in this post) as no one was completely right or completely wrong.

This whole discussion seems to me to be a rather silly argument. Energy is not destroyed when work is done. It just goes somewhere else. The concept of heat as a substance was replaced by the concept of 'internal energy of a system'. All systems have internal energy. When they do work or transfer heat this becomes an increase in the internal energy of another system (generally called the surroundings). This increase in the internal energy of the surroundings is matched by (equal to) a corresponding decrease in the system doing the work or transferring the heat. Energy itself is not a substance, it is a property of a system. Internal energy can be held within a system in several different ways, which we distinguish as different 'forms' of 'energy'. This concept leads to one version of the First Law as The energy of an "isolated system is constant", which is consistent with the more oft quoted dE = q+w. @Tom Booth I suggest if you genuinely want further understanding then you study the terms open, closed and isolated; energy, internal energy, potential energy, mechanical energy, electrical energy, so that you can put these concepts into their proper place.

Did I ? Please quote the passage since this forum insists on not numbering the posts. In order to show that some set S constitutes a mathematical Field it is necessary to prove that S and its elements satisfy each and every one of the 11 Field axioms. https://www2.math.upenn.edu/~kazdan/202F13/notes/FieldAxioms.pdf If it is a field with extra properties these are extra axioms that will also need proof. But remember each and every axiom must be proven satisfied.

I don't wish to enter a person slanging match with anybody. So lets clear up the stagnation pressure once and for all. I have indicated the relevent theory and the factor of 1/2. So Seth's formula is the overpressure above normal stram pressure, which has not been included

That's why I have given up on this thread a long time ago. +1

Perhaps you would like to elaborate on this wall of yours, and what you actually want to know. The dam wall I was thinking of is 90metres high and 500 metres wide. Wind loading analysis can be very complicated and what I offered was the beginning of it to try to connect the quantities you mentioned without calculus, for understanding purposes. Winds are normally specified by a general incident pressure and a host of shape and other factors that control the wind regime. The resultant load is of course forces and/ or moments. I say moment because it may be that for a brick wall the wind pressure can cause unwanted tension in the brickwork, which cannot support tension.

? ? Why indeed I don't recall mentioning them. You don't seem to have anything to say about the bog standard fluds calculation.

Every fluid element is travelling at the winf speed v, until it is brought to rest by the wall surface. Therefore every element has the full mass x velocity momentum There is no averaging factor of 1/2 for momentum, only kinetic energy.

Bernoulli is an energy equation. Unless the wall moves, which we are not considering here, there is no energy loss by the wind 1/2mv2 is conserved. Energy, pressure and volume are of course all scalars.